1. Field of the Invention
The present invention relates to an electrode material for a secondary battery, and more particularly to a sulfide series electrode material.
2. Description of the Prior Art
In recent years, demands for portability of communication appliances and OA (Office Automation) appliances have intensified competition for lightweight and miniaturized products. Correspondingly, a secondary battery which is used in such an appliance or as a power source for an electric vehicle requires high efficiency. Under such circumstances, various kinds of batteries using new electrode materials have been developed. Among them, an electrode material using a disulfide compound (hereinafter referred to as "disulfide electrode material") has been noticeable because of its relatively high energy density as disclosed in U.S. Pat. No. 4,833,048, the disclosure of which is incorporated herein by reference. For example, a sulfide compound having a triazine ring or a thiadiazole ring has been used as an electrode material.
Assuming that the disulfide compound is represented by R--S--S--R (R denotes an organic functional group), the disulfide bond (S--S bond) is cleaved by supply of two electrons by electrolytic reduction. It is combined with a cation or proton (M.sup.+) in an electrolytic solution to provide a salt represented by 2 (R--S.multidot.M.sup.+). The salt is returned to original R--S--S--R by electrolytic oxidation to discharge two electrons. The secondary battery is expected to have an energy density of 50 Wh/kg or more, which is approximately equal to that of other ordinary secondary batteries.
However, as reported by the inventors of the above U.S. Patent in J. Electrochem. Soc., Vol. 136., No. 9, pages 2570-2575 (1989), the electron moving speed in an electrode reaction of the sulfide-series secondary battery is very low so that it is difficult to take out a large current for practical use at room temperatures. The above sulfide-series secondary battery is limited for use at 60.degree. C. or higher.
As a technique for improving the sulfide-series secondary battery so as to deal with a large current, as disclosed in JP-A-5-74459, an electrode material in which an organic compound having a thiadiazole ring and a disulfide group are combined with a conductive polymer, such as polyaniline, has been proposed.
However, the secondary battery according to the prior art can increase the current because of an increase in the reaction speed, but it cannot improve the energy density; rather, the energy density is lowered.